A451 - Stellar Astrophysics - Introduction to Linux
Most astronomers using PC computers use the Linux operating system for data reduction
and analysis. While not (quite) as user friendly as Windows, Linux offers the user more
"hands on" control of what the computer is doing, and much of the software that
astronomers need runs under Linux.
We will use Linux computers for A451. This guide is designed to help you become
familiar with the Linux operating system and make the transition from Windows to
Linux. Many of the basic functions of Windows (word processing, spreadsheets,
browsers, etc.) are also available under Linux, and many of them operate in the same
basic way.
Technically, Linux refers to the operating system and/or its philosophy. The user
interface in Linux is called a shell because it separates you from the inner guts of the
system. Mostly, the shell just runs the programs you tell it to run. Almost all the
commands you will use are just regular programs -- the shell doesn't care if it's a program
you wrote or one that came with the system.
We carry out tasks on Linux computers using two different types of interfaces. These are
the text based interface and the graphical interface. Each has advantages and
disadvantages. You should choose the interface most suitable to the job you need to do.
The command line interface is more suited to complex tasks where presenting the work
visually is impractical. On the other hand, the graphical interface works well for
problems that are strongly visual or can be presented in a menu format. In this class, you
will need to be able to work with both interfaces.
We will use the Gnome graphical interface. It is one of several choices available, but all
are rather similar and if you are familiar with one you will be able to find your way
around the others.
Logging in and out
To login in, type: your username
Enter the password: (nothing will appear as you type this)
On a Linux workstation, you will need to start the window system after you log in. The
window system has two modes: a high resolution mode and a low resolution mode. Most
of the time you will want the high resolution mode, so you should type
startx
In the top left corner of the screen you will find icons for your home directory and the
trashcan. Double click on the home directory icon to see the contents of your home
directory or other directories in graphical format. Files that you remove using the
graphical interface tool will appear in the trashcan. Remember that files you remove
while using the shell are gone forever and will not appear in the trashcan – you should be
very careful about deleting files.
In the bottom left corner, you will find buttons for accessing applications. The buttons
are:
 A red hat, which gives you access to the program menu. This is similar to the
Windows Start Menu.
 A globe with a computer mouse, which starts the Mozilla web browser
 A notepad and pencil, which starts the GEdit text editor
 A computer screen, which provides opens an XGTerm terminal window
 A magnifying glass, which opens ds9, a tool to display astronomical images
 A piechart and ledger, which opens the Gnumeric spreadsheet application
When you are ready to log out, click on the red hat, then choose “Log Out”. A window
will pop up to confirm that you want to log out. Choose “OK”. You will be returned to
the terminal. Type
logout
to log out from the terminal. The login prompt will appear again. Always remember to
log out completely when you are finished!
Command Line Window Applications
In the graphical interface, you can run a program by pressing ALT+F2. A dialog box will
appear, and you may type the command you would like to run. You may also use a shell
terminal such as XGTerm or XTerm. These applications will provide you with a terminal
inside a window. The button at the bottom left corner of the screen allows you to open
XGTerm, or you can type “xterm” or “xgterm” into the run dialog. You can also run
XTerm from the program menu.
In XGTerm, the font size should be set to “huge” by default. If you prefer a different
size, you can change it by pressing and holding control and clicking the middle mouse
button. When you do this, a menu will appear offering you a choice of sizes.
You can use the XTerm and XGTerm in just the way you would if you weren’t using the
windowing system, and you can also run graphical applications from an XTerm or
XGTerm window. When you run a graphical application this way, it’s usually a good
idea to run the program “in the background” so that you can continue to use the terminal
window. To run a program in the background, type an ampersand (&) after the
command. For example,
xterm &
Directories
Linux "folders" are called directories. The top-level, root directory is called /. Your
home directory is /home/username. From anywhere you can get back there by typing
simply
cd
The short-hand name for the directory you happen to be in at any time is called "." and
the directory in which the current directory resides is called "..". Typing "cd .." will
move you to the next higher level directory. Several useful commands for directories are
listed below.
Command
cd
pwd
mkdir
rmdir
ls
Function
Change directory
Print working directory
Make a new subdirectory
Remove a directory
List files in a directory
Examples
cd, cd .., cd /home/catyp
pwd
mkdir newdirectory
rmdir emptydirectory
ls, ls –l
Files:
Files reside in directories. Use the ls command (or ls -l for more information) to see all
the files in a directory. Useful commands for manipulating files include:
Command
ls
mv
cp
Function
List files
Rename (move) a file
Copy a file
rm
Delete (remove) a file
cat
Output the contents of a file
to the screen
Identify the type of file
file
Examples
ls, ls -l, ls dirname
mv oldname newname
cp oldname newname
cp oldname dirname/
rm filename
rm file1 file2 file3
rm -r dirname
cat filename
file filename
Editing Files
You will frequently need to edit text files when you are working in Linux. There are a
number of editors to choose from. You can open the editor simply by typing its name on
the command prompt.
When working in the windowing system, you can use GEdit, a simple graphical editor.
To use GEdit, type
gedit &
at the command line or click the notepad icon at the bottom left corner of the screen. A
window will appear with an interface similar to Windows Notepad. Open and save your
work using the buttons above the command line. You can also type
gedit filename &
to open a file in GEdit.
Many other editors are available, including EMACS, Pico, and vi, that are sometimes
useful in other applications. Documentation for these editors is available in room 311.
Printing
You can send a plain text file, a postscript format file, or a PDF file to the printer by
typing
lpr filename
To print other types of files, you will need to view them with the appropriate application
and print them from there.
Shortcuts
Cutting and Pasting:
In the windowing system and on the text interface, it is possible to cut and paste text.
First, select the text with the mouse. Once you select the text, it will automatically be
copied to the clipboard. You can now paste the text elsewhere by clicking the middle
mouse button. A duplicate of the text will now appear at the text cursor position.
Some Linux applications also have a Windows style clipboard available. To copy to this
clipboard, press CTRL+C. Then, select your paste location and press CTRL+V.
Wildcards:
If you need to refer to a list of multiple files, you can use wildcards. These are
characters that have a special meaning to the shell. The two most important wildcards are
the asterisk (*) and question mark (?). An asterisk will match any string of zero or more
characters in a file name, while the question mark will match any single character in a file
name. For example:
The pattern…
f*
*.txt
f?.txt
*.*
Will match…
f1, f2, f3, fruit.txt
somefile.txt, pasta.txt
f1.txt, f2.txt, fz.txt
fruit.txt, pasta.txt,
pasta.txt.bak
But not…
somefile.txt, 1f
pasta.txt.bak, somefile
f1, f1.txt.bak, f.txt
f1, somefile
Linux "help"
If you don’t know how to do something, Linux has a built-in help system that you can
use. The most important tools are listed below.
man command
apropos keyword
whatis command
Display the manual page for a command.
Print a list of commands apropos to a
keyword with a one-line explanation for
each command.
Print the one-line explanation for a
command.